A novel temperature-controlled laser system to uniformly activate cutaneous thermal receptors during movable thermal stimulation.

Abstract

<i>Objective</i>. Laser stimulators have been widely used in pain studies to selectively activate A<i>δ</i>and C nociceptors without coactivation of mechanoreceptors. Temperature-controlled laser systems have been implemented with low-temperature variations during stimulations, however, these systems purely enabled stationary stimulation. This study aimed to implement, test and validate a new laser stimulation system that controls skin temperature by continuously adjusting laser output during stimulus movement to allow accurate investigation of tempo-spatial mechanisms in the nociceptive system.<i>Approach</i>. For validation, laser stimuli were delivered to the right forearm of eight healthy subjects using a diode laser. The laser beam was displaced across the skin to deliver a moving thermal stimulation to the skin surface. To test the function and feasibility of the system, different stimulation parameters were investigated involving two control modes (open-loop and closed-loop), three displacement velocities (5, 10 and 12 mm s^-1), two intensities (high 46 °C and low 42 °C), two stimulus lengths (20 and 100 mm) and two directions (distal and proximal).<i>Main results</i>. During closed-loop control, the stimulation error and variation of stimulation temperatures were significantly smaller than during open-loop control. The standard deviation of stimulation temperatures increased significantly with stimulation intensity and displacement length.<i>Significance</i>. This study showed that more accurate, less variable laser stimulations were delivered to the skin using closed-loop control during a movable stimulus. The more uniform skin temperature during stimuli is likely to ensure a more uniform nociceptor activation.